Despite common assumptions, seaweed farms really shine at removing and storing CO2
Seaweeds are versatile algae. They are sources of food, medicine, and many other products, and they have the added benefit of being extremely efficient at removing CO2 from the atmosphere as they grow.
However, seaweed aquaculture's potential for sequestering carbon is overshadowed by the assumption that the biomass will be easily converted back into CO2, says UConn Department of Earth Sciences Assistant Professor Mojtaba Fakhraee. Fakhraee and co-author Noah Planavsky of Yale University argue this is not the case, and we need to reconsider the carbon removal potential of these dynamic systems. Their research is published in Nature Communications Sustainability.
Fakhraee explains that coastal seaweed farms are an extremely effective way to remove CO2 from the atmosphere because these algae sequester carbon at high rates, "this nature-based technology removes CO2 and converts it to biomass, but one of the main challenges discussed is that it is expected that most of the carbon and biomass produced would eventually be used by microbes in the water or in the sediment to produce CO2. That was the main concern, whether these are actually a good way of capturing carbon, or not."
The researchers wanted to explore if this was a real concern, and they happened upon an overlooked process that takes place in the sediment beneath seaweed farms, says Fakhraee,
"In this paper we highlighted the fact that these algae farms drive a climate-friendly feedback. This feedback is the production of alkalinity with bicarbonate chemical species that can eventually change the chemistry of the water and shifts the pH and the whole equilibrium of the CO2 within the water," says Fakhraee.
This works because seaweed farms speed up the process of building a layer of sediment as organic matter sinks to the seabed. These sediments create low or no-oxygen (anaerobic) environments where microbes take the carbon-rich biomass and produce bicarbonate. The bicarbonate then acts as a buffer to produce more alkaline, or less acidic, conditions. This is crucial because under oxygenated (aerobic) conditions, microbes use different pathways to consume the organic matter, which includes the production of CO2.
"Bicarbonate is like a magic chemical agent for changing the chemistry of the water because it shifts the pH, and because there is an increase in the amount of organic matter from the seaweed, it increases the rate of production of bicarbonate," says Fakhraee. "This would eventually result in a sequence train that removes CO2 from the atmosphere. That chemical process was not considered or was largely overlooked by previous studies."
The researchers wanted to explore the production of bicarbonate from anaerobic respiration and calcium carbonate dissolution beneath seaweed farms, says Fakhraee, and they used a model that tracks the fate of organic carbon in the sediment beneath the seaweeds to demonstrate how these are ideal systems for this carbon sequestration process.
An important feature of the bicarbonate production is that, even if organic matter is reliably stored in the sediment, there is always the chance that it could be dislodged and made available for microbes to process and release the biomass as CO2. However, if organic matter is used to produce bicarbonate, it is a more permanent type of carbon capture, and the shift in the water's chemistry is long-lasting, perhaps on the scale of thousands of years, says Fakhraee.
Using the model and estimates from around the globe, there are currently approximately 3.5 million hectares of seaweed aquaculture, potentially sequestering as much as seven million tons of CO2 annually. Fakhraee says the acreage is projected to be larger and the industry will likely grow substantially in the coming years, and also scale up the carbon sequestering capacity of this agricultural practice.
"It's very sustainable, it doesn't require too much technology, and there is not much controversy around using seaweed as a source of food opposed to the other sources of protein concerning the production of greenhouse gasses and other things," says Fakhraee. "I would say interest in the investment toward this type of farming will grow."
Fakhraee says they didn't expect the scale of carbon capture to be as significant and for the numbers to be comparable to other coastal ecosystems like mangroves and sea grasses.
"Seaweed farms sequester a bit more than sea grasses, and they are on par with mangroves and some other basic types of the blue carbon ecosystems. It is quite surprising to know that there is a huge potential for this ecosystem to sequester carbon but at the same time, unlike the other types of blue carbon ecosystem that they have with a wide range of ecosystem services, for sure, they also have a large list of benefits for people."
To get a more comprehensive understanding of this newly realized benefit of seaweed farms, Fakhraee says it is important to quantify what is happening by taking large-scale measurements. This will give insights into factors that drive different elements of the carbon capture process, for instance if there are seasonal changes or other factors that influence the extent of the carbon captured in the system.
"This needs to be done. Our study was just trying to push this idea further and transform the discussion. This type of nature-based technology for carbon capture should be more interesting and attractive to people, that's the hope that we have from this paper," says Fakhraee.
There are also important economic implications for this finding, Fakhraee explains, for example when it comes to carbon sequestration, it opens up the potential for monetization through means like carbon credit trading. Since seaweed aquaculture is an established industry with growing interest, this additional economic opportunity may make the industry even more attractive and drive further investment.
"There's another part of this story which has to be recognized and needs to be studied more to be able to understand that better," says Fakhraee. "Seaweed farms are not just about the food they are a reliable means of carbon capture too."
